A typical wireless communication system includes a number of base stations each providing coverage in which to serve user equipment devices (UEs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped devices, whether or not user operated. In turn, each base station may sit as a node on a core access network that includes entities such as a network controller, switches and/or gateways, and the core network may provide connectivity with one or more external transport networks such as the public switched telephone network (PSTN) and/or the Internet. With this arrangement, a UE within coverage of the system could engage in air interface communication with a base station and could thereby communicate via the base station with various remote network entities or with other UEs served by the base station.
Such a system could operate in accordance with an radio access technology (RAT), examples of which include, without limitation, Long Term Evolution (LTE) (using orthogonal frequency division multiple access (OFDMA) and single-carrier frequency division multiple access (SC-FDMA)), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), Global System for Mobile Communications (GSM), IEEE 802.11 (WIFI), and BLUETOOTH. Each RAT could define its own procedures for registration of UEs, initiation of communications, handover of UEs between base station coverage areas, and other functions.
Per the RAT, each base station coverage area could define an air interface for carrying communications between the base station and UEs, including a downlink from the base station to UEs and an uplink from UEs to the base station. In practice, the air interface could occupy a carrier, which could be frequency division duplex (FDD), having separate ranges of frequency for downlink and uplink communication, or time division duplex (TDD), having a single range of frequency multiplexed over time between downlink and uplink use. Further, the air interface could be divided over time into transmission time intervals (TTIs), and communication on the air interface could occur on a per-TTI basis.
Under a representative RAT, the air interface could define various resources and channels for carrying particular communications. On the downlink, for instance, the air interface could define a reference channel carrying a broadcast reference signal that UEs can measure to evaluate downlink coverage quality, the air interface could define various other downlink control channels for carrying control signaling to UEs, and the air interface could define one or more downlink traffic channels with resources allocable on an as-needed basis to carry bearer data (e.g., application-layer communications) and other information to UEs. And the uplink, the air interface could define an access channel for carrying UE access requests to the base station, the air interface could define various other uplink control channels for carrying control signaling to the base station, and the air interface could define one or more uplink traffic channels with resources allocable on an as-needed basis to carry bearer data and other information to the base station.
When a UE initially enters into coverage of such a system, the UE could scan for a best base station coverage area in which to operate, and the UE could then engage in signaling with and via the base station, to register for service. For instance, the UE could evaluate the reference signal from each available coverage area to identify the coverage area with the best downlink coverage (e.g., downlink receive signal strength such as reference-signal receive power (RSRP) or received signal strength indicator (RSSI), or downlink signal-to-noise ratio such as reference-signal receive quality (RSRQ), signal-to-interference-plus-noise (SINR), C/I, Ec/Io, etc.) And the UE could then engage in signaling with the associated base station, to register for service.
The UE could then be served by the base station in a connected mode or an idle mode. In the connected mode, the UE would have an established radio-link-layer connection with the base station through which the UE and base station could exchange bearer data, facilitating communication by the UE on an external transport network, for instance. And in the idle mode, the UE would not have an established radio-link-layer connection with the base station but could periodically check for paging from the base station and could transition to the connected mode when necessary to engage in bearer communication.
When so served, the UE could also regularly monitor the quality of downlink coverage from its serving base station and could periodically report measured downlink coverage level (e.g., downlink receive signal strength and/or downlink receive signal quality) to its serving base station. And if and when the base station determines that the UE's measured downlink coverage level is threshold low, the base station could coordinate handover of the UE to another coverage area. Further, if and when the UE detects that the downlink coverage level is threshold low, the UE could responsively transmit a measurement report to its serving base station, to trigger possible handover of the UE to another coverage area.
For this purpose, the base station could apply a downlink coverage threshold value (e.g., threshold downlink receive signal strength and/or threshold downlink receive signal quality) to trigger handover processing of the UE. For instance, the base station could itself be provisioned with the downlink coverage threshold value and could be configured to determine when the UE's reported downlink coverage drops to that threshold level and to responsively take action to facilitate handover of the UE (e.g., then directing the UE to report adjacent downlink coverage and/or coordinating handover of the UE to adjacent coverage) Further, the base station could provision the UE with the downlink coverage threshold value, directing the UE to determine when the UE's downlink coverage drops to that threshold level and to responsively provide a measurement report to the base station to facilitate possible handover of the UE.